2-1 Electric Circuit Basics

Static vs. Current Electricity

So far we've been looking at static electricity. While static electricity is a lot of fun and is useful in some situations, current electricity is something you use every day of your life. In fact, your life would be a LOT different if it didn't involve current electricity! Let's compare them.

As you can see, there are some big differences. But both types of electricity carry energy from one place to another. Energy, formally defined as "the ability to do work," is something that shows up, either directly or indirectly, in every part of every Science course you'll ever take.

An electric circuit is a complete round-trip that charges can take, and these charges carry electrical energy with them. This electrical energy often gets changed into another form, which is useful to us in some way: heat, light, sound, or movement.

A Sample Electric Circuit

Let's take a look at an average room with lights on the ceiling and a light switch on the wall. It's such a common situation that you probably don't think much about it, but it involves current electricity in every part of it.

Looks pretty normal, right? But there's a lot more buried within the walls and above the ceiling that you might not know about. Here's a peek behind the scenes:

The red lines are wires, also called conductors. These are ususally made of metal, and allow charge to easily flow through them. Think of them like roads, and the chares are like cars driving on those roads.

But, there's more to this room:

We've labelled a load, which converts the energy the charges are carrying into something else. In this case, the load is a light bulb: it converts the energy carried in the wires into light energy (and some heat, but that's not what we really want here).

There's also a switch, which controls the flow of charges easily. Technically you don't need a switch, but it makes things much easier. Without a switch, every time you wanted to turn out the light, you would have to go up to the ceiling and remove the light bulb. Good thing there's a switch on the wall!

One final thing needs to be said here... the light energy that comes of the bulb needs to come from somewhere. It's like the old saying: "You can't get something from nothing." So, the energy needs to come from somewhere else: we call this a source of energy. In this case that's a power plant of some sort, whether it's nuclear-powered, hydroelectric-powered, solar- or wind-powered, or some other form of energy generation. We will often be using cells or batteries as our source of energy in our labs.

Examples of Electric Circuits

Flashlight

Here's another circuit that has a light bulb as its load. But in this case, the source isn't power inside the walls of your home: it's contained right inside the flashlight itself, in the form of a battery.

All of the parts are there, in a very small container. Some flashlights use tiny batteries that are the size of a quarter, and a miniscule LED light which is extremely bright for its size.

No matter the size or shape, that circuit is going to have the four parts that we've described above.

Solar-powered calculator

Calculators are very handy devices. They can also take a very small amount of energy, which can be provided by a solar cell: a special piece of silicon that converts light (from either the sun or the lights in a room) directly to electrical energy.

The solar cell provides enough electricity to power the small, computer-like device inside. Often, a solar-powered calculator will also have a battery inside it to give it energy if there isn't enough light in the room.

If you cut away inside a calculator, you might see something like this:

Here you can clearly see the cell inside, and the wires connecting it to the green circuit board. (This means the calculator is not necessarily solar-powered. But this cell might be just a backup, in case the solar cell doesn't have enough light.) The circuit board, and the things on it, are what makes the calculator work, and is the load here. The "brain" of the calculator, a microchip, is sealed inside the shiny black circle.

Here's a very old calculator, which is powered by 3 AA cells:

As technology has become more advanced, electronics have become much smaller, lighter and energy efficient. This calculator may have been very advanced (and expensive) the 1970s, but you can buy something much more powerful and efficient than this for a few dollars today.

Circuit Diagram Symbols

Things like cells, switches, wires and loads can look very different, even if they all do the same thing. Because this could be confusing, scientists and engineers around the world use a set of symbols that everyone has agreed on, if you want to show how a circuit is put together. We call these circuit diagrams.

Here's a photo of simple electric circuit with a cell, a switch, a load (a light bulb), and wires:

Now, let's look at the circuit, side-by-side with their circuit diagram symbols:

The cell is at the top, and the + and - terminals are labelled. The switch is on the right, the bulb is at the bottom, and the wires are shown as straight lines that turn at 90-degree angles. (This makes the diagram look neat and clean, and easier to read.)

When a switch is open, current won't pass through: there's a break in the circuit, and the current can't flow through. When a switch is closed, current can go through. We draw closed and open switches differently:

Cells and Batteries

What do you call the thing below?

Most people, including science teachers, mistakenly call this source of information a "battery." But it's not: it's a cell. A battery is a collection of cells, all connected together. When we build circuits, we need to be careful how we draw the source, and what we call it:

A common size of battery you can buy in stores is a "9 volt" battery -- it's rectangular and has both of the terminals (the + and - connectors) on the top. It is actually a battery, not a cell, and if you cut it open you would see the six individual cells inside.

These cells are actually "AAAA" size -- smaller than the AAA-sized cells that you commonly see in small electronic devices like TV remote controls.

Practice Questions

The Basics

1. List the four main parts of any electric circuit.

2. Which part of an electric circuit is optional?

3. Draw an electric circuit, using proper symbols, showing a 3-cell battery, a switch, and two light bulbs one after the other. (You can show the switch either open or closed.)

Extension

1. If you have some small screwdrivers, take apart a small electronic device. (Make sure it's a device you wouldn't mind losing, if you can't get it back together.) Can you find the different parts of the circuits inside it?